Embodiments of the present invention relate to a cooling system for cooling a motorcompressor unit for processing a working fluid.
The cooling system of embodiments of the present invention is particularly conceived for improving the efficiency of motorcompressor for subsea applications, but any other motorcompressor may be considered.
Integrated motorcompressor units here considered comprise, integrated in a casing, a motor and a compressor.
Generally a motorcompressor unit of the type here considered comprises a centrifugal compressor processing a process gas, the compressor being arranged in a housing together with a motor, usually consisting of an electric motor.
The compressor of the motorcompressor unit could be fluidly connected with an external separator machine placed between the well and the inlet of the unit. A separator device is present also inside the casing at the inlet of the compressor.
The motorcompressor unit of the kind of embodiments of the present invention comprises a motor which drives the compressor via a shared rotating shaft supported on each end by magnetic bearings. Said shaft connect the rotor of the electric motor and the rotor of the centrifugal compressor on which are installed the impellers of the compressor, said shaft usually does not project outside the casing(s). The compressor generates a flow of compressed process gas.
When used to directly drive a compressor, such as a centrifugal compressor, the shaft is required to rotate at relatively high speeds. In addition to the heat generated by the electrical loss mechanisms that are characteristic of electric motor drivers, operating the motorcompressor device at high speeds increases windage frictional losses generated by the rotating components.
Motorcompressor units used in the production or transport of hydrocarbons are provided with a shared rotating shaft supported by a rotor-bearing system.
In case of electric motor, heat is also generated by the electrical systems that are characteristic of electric motor drivers. Heat is also generated through the windage friction resulting from the rotating components operating in pressurized gas.
If this heat is not properly dissipated, it negatively affects the performance of the motor and can damage the insulation of the stator. Increased temperatures can also adversely affect the rotor-bearing systems of both the compressor and motor, thus leading to bearing damage and/or failure.
For cooling the motor and bearings in a motorcompressor unit, is provided a cooling circuit which may be an open loop cooling circuit or a quasi-closed-loop cooling circuit where gas is drawn from the process stream at some point in the compression process.
An example of such cooling circuit is shown in FIG. 1.
Only a small amount of process gas is fed into the cooling circuit from the process stream. The quasi-closed-loop cooling circuit often uses a small blower to circulate the cooling gas through the cooling circuit. In subsea applications, the cooling gas is typically cooled in a sea water-cooled heat exchanger.
This process gas is then passed through the motor and bearing areas to absorb heat.
According to the current art, motorcompressor unit, in particular motorcompressor for subsea applications, uses as cooling media the process gas which may be cooled through an external cooler.
In these applications the cooling gas may be circulated in a quasi-closed loop: the process gas of the compressor is used to cool the bearing of the rotary shaft positioned at the compressor and the intermediate diaphragm positioned between the motor and the compressor.
The process gas then enters the motor area where a blower pressurizes the gas and forces it to flow into cooling ducts, thus cooling the bearings provided inside the motor area and the motor itself. The process gas is then circulated through an external cooler where is cooled.
When the machine works at low-medium pressure, the cooling efficacy is still good using the same process gas handled by the machine in a quasi-closed loop described above. When the machine works at high pressure, the cooling efficacy of the process gas would be higher due to the increasing of the gas density, but on the other hand, over a certain level of pressure, the windage losses of the motor becomes very high due to the gas density, consequently a very high rate of the electric power which operates the motor is lost for moving the process cooling gas inside the motor area of the machine, and the cooling method becomes ineffective.